Matched filter for scalable spread spectrum communications systems

ABSTRACT

A spread spectrum communications system uses long, scalable PN sequences to achieve variable communication rates. A low-complexity and scalable matched filter architecture is used to provide a large processing gain and robust recovery of multiple devices in long reach, high ambient-noise environments. The spread spectrum communications system includes a transmitter and a receiver. The receiver includes an RF receiver and a first matched filter receiving data from the RF receiver. A plurality of phase/frequency shifters receives a signal from the first matched filter. A plurality of second matched filters receive data from the plurality of phase/frequency shifters. An equalizer/decoder receives signals from the plurality of phase/frequency shifters.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to spread spectrum communications. Morespecifically, this invention relates a filter and use of long, scalable,separable PN sequences to achieve variable communication rates togetherwith low complexity in spread spectrum communications.

2. Description of Related Art

A variety of spread spectrum communications systems are well known inthe art. Often these systems use very long PN codes to achieveprocessing gain. However, typically, such prior systems have substantialproblems with coherence and frequency error and such problems interferewith achieving the objective of robust spread spectrum communications.Typically, these prior systems also require large matched filters todespread signals, where the use of correlators is inappropriate.

Although the following cited U.S. patent documents are not necessarily“prior art,” the reader is referred to the following U.S. patentdocuments for general background material. Each of these patents ishereby incorporated by references in its entirety for the materialcontained therein.

U.S. Pat. Nos. 4,351,064; 4,953,178; 4,977,578; Re. 33,875; 5,319,672;5,400,359; 5,471,509; 5,737,368; 5,740,096; 5,790,588; 5,790,590;5,815,055; 5,881,099; 5,909,461; 5,912,644; 5,926,512; 5,974,082;5,991,332; 6,031,415; 6,104,746; 6,128,332; 6,154,482; 6,154,487;6,212,219; 6,233,272; 6,265,807; 6,317,452; 6,331,998; 6,333,925;6,356,555; 6,434,185; 6,493,334; 6,493,376; 6,549,567; 6,556,621;6,560,270; 6,560,271; 6,567,017; 6,570,865; 6,580,750; and 6,590,881.

SUMMARY OF INVENTION

It is desirable to provide a method and system for the use of long,scalable PN sequences in long distance spread spectrum communicationssystems. Moreover it is also desirable to provide a low-complexitymatched filter architecture that provides robust recovery of multipledevices in a long reach, high ambient noise environments.

Accordingly, is an object of this invention to provide a method andsystem for long distance spread spectrum communications systems thatmakes use of long scalable PN sequences. Moreover, it is an object ofthis invention to facilitate operation in high ambient noiseenvironments, by increasing processing gain, to thereby provide thecapability to trade off data rate for increased robustness in such highambient noise environments.

Another object of this invention is to provide a method and system forlong distance spread spectrum communications system that includes amatched filter with reduced complexity that allows robust recovery ofmultiple devices in long reach, high ambient noise environments.

A further object of this invention is to provide a method and system forlong distance spread spectrum communications systems that provides forscalable data rates. A still further object of this invention is toprovide a method and system for long distance spread spectrumcommunications systems that does not require the locking of the receiverto an individual client device.

A still further object of this invention is to provide a method andsystem for scaling the complexity of the receiver, including the matchedfilter structure, to receive and demodulate transmissions from devicespossessing very different frequency tolerances.

Another further object of this invention is to provide a method andsystem for determining the time-of-arrival of a received signal.

Additional objects, advantages, and other novel features of thisinvention will be set forth in part in the description that follows andin part will become apparent to those of ordinary skill in the art uponexamination of the following, or may be learned with the practice of theinvention as described herein. The objects and advantages of thisinvention may be realized and attained by means of the instrumentalitiesand combinations particularly pointed out in the appended claims. Stillother objects of the present invention will become readily apparent tothose skilled in the art from the following description wherein there isshown and described the preferred embodiment of the invention, simply byway of illustration of one of the modes best suited to carry out thisinvention. As it will be realized, this invention is capable of otherdifferent embodiments, and its several details and specific circuits arecapable of modification in various aspects without departing from theinvention. Accordingly, the objects, drawings and descriptions should beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification, illustrate present preferred embodiments of theinvention. Some, although not all, alternative embodiments are describedin the following description. In the drawings:

FIG. 1 is a system block diagram showing the major components of onepreferred embodiment of the invention.

FIG. 2 is a detailed block diagram of the matched filter architecture ofone preferred embodiment of the method of this invention.

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

DETAILED DESCRIPTION

This invention is a filter designed specifically for scalable spreadspectrum communications systems and cooperative techniques for the useof long, scalable Pseudo-Noise (“PN”) sequences for variablecommunication rates. Link distance and performance margins in spreadspectrum communications can be increased by increasing processing gain.Processing gain is itself directly related to the length of the PN codeemployed. For example, where the data rate is constrained to one bit perPN sequence, a PN code length of 1000 yields a processing gain of 30 dB,while a PN code length of 10,000 yields a processing gain of 40 dB. Inthe present invention, long, scalable PN sequences are used with alow-complexity matched filter architecture to provide variablecommunication rates, robust recovery of multiple devices in ordinary aswell high ambient noise operating environments. In the present inventionsites may receive signals, simultaneously or near simultaneously, from amultitude of transmitting devices. Each of these received signals willtypically have a corresponding frequency error. Furthermore, thefrequency error of the devices may be very different depending on systemdesign objectives, which may be selected to optimize the tradeoffsbetween variables such as system quality, complexity, cost, and service“class” of the devices. These system requirements tend to precludelocking the receiver to an individual transmitting “client” device inorder to achieve frequency coherence across long PN sequences. Thisinvention addresses this problem with a receiver architecture designedso as to facilitate the “simultaneous” receipt of signals, with a widerange of data rates and a wide range of frequency errors, from a varietyof devices.

Referring now to FIG. 1 shows a system block diagram illustrating themajor components of one preferred embodiment of the invention. Data bits100 are spread 101 by a variable length PNA code 102. Typically, thisspread 101 operates to spread the data 100 from 1 to 1023 chips,although in alternative embodiments the spread may be varied withoutdeparting from the concept of this invention. The resulting chips 106are further spread 103 by a fixed length PNB code 104. In the presentpreferred embodiment of the invention, a chipping rate of 5M CPS isused, further scaling the data rates from 19.608 kbps to 38 bits/second,providing processing gains of from 24 dB to 51 dB, respectively,depending on the application requirements and the availablesignal-to-noise ratio in the available link (communication channel). Theresulting spread data is transmitted, typically using a radiotransmitter 105 for use with an RF link. Table 107 shows the performanceof a variety of PNA code lengths. As can be seen, with a constant chiprate, here 5M CPS, as the PNA code length is increased from 1 to 511,the chips per bit increase from 255 to 130305, the processing gainincreases from 24 dB to 51 dB and the data rate is reduced from 19608 to38 bps. Typically, the length of the fixed length PNB code is designedto be short enough to ensure that the worst case frequency error plusthe Doppler shift will cause no more than 180 degrees of phase roll, ora correlation loss in the first matched filter of approximately 4 dB.

FIG. 2 shows a detailed block diagram of the matched filter architectureof one preferred embodiment of the method of this invention. Thereceiver portion of the present data link of this invention includes aradio receiver 201. The output 202 of the receiver 201 provides I & Qchannels, which are first processed by a matched filter 203. The matchedfilter 203 uses the PNB codes as coefficients. Because the frequency canbe rolling as much as plus or minus one-hundred eighty degrees across asingle PNB code length, a bank of frequency shifters 204, 205, 206, 207,208, 209, 210, 211 are used prior to sending the filtered data 222through the PNA matched filters 212, 213, 214, 215, 216, 217, 218, 219,220. Although, in this FIG. 2 eight frequency shifters and nine matchedfilters are shown, the number of each actually employed in a particularembodiment of the invention is dependent on the worst-case frequencyroll and the allowable phase error across the concatenated PN codes. Thefrequency shifters 204, 205, 206, 207, 208, 209, 210, 211 perform acomplex frequency shift. The matched filters 212, 213, 214, 215, 216,217, 218, 219, 220 receive data from the frequency shifters 204, 205,206, 207, 208, 209, 210, 211 and the matched filter 203 and provide afiltered output signal received by the equalizer/decoder 221. Theequalizer/decoder 221 examines the outputs of each matched filter 212,213, 214, 215, 216, 217, 218, 219, 220 and determines the frequencyshift of the received signal and appropriately selects the set of mostadvantageous signals.

It is also notable that two smaller matched filters, rather than onelarge filter is used presently to despread the spread signal. Thismeans, that in the preferred embodiment of this invention, a single longmatched filter can be avoided, thereby considerably reducing the amountof signal processing hardware that is required.

The described embodiment of this invention is to be considered in allrespects only as illustrative and not as restrictive. Although specificsteps and associated formulas are provided, the invention is not limitedthereto. The scope of this invention is, therefore, indicated by theclaims rather than by the foregoing description. All changes, which comewithin the meaning and range of equivalency of the claims, are to beembraced within their scope.

1. A spread spectrum communications system, comprising: (A) atransmitter, said transmitter further comprising: (1) a data source; (2)a first multiplier/mixer spreading data from said data source with afirst pseudo noise source; (3) a second multiplier/mixer spreading datafrom said first mixer with a second pseudo noise source; (4) an REtransmitter; (B) a receiver, said receiver further comprising: (1) an REreceiver; (2) a first matched filter receiving data from said REreceiver; (3) a plurality of phase/frequency shifters, receiving asignal from said first matched filter; (4) a plurality of second matchedfilters receiving data from said plurality of phase/frequency shifters;and (5) an equalizer/decoder receiving signals from said plurality ofphase/frequency shifters.
 2. A spread spectrum communications system, asrecited in claim 1, wherein said first pseudo noise source provides avariable length code sequence.
 3. A spread spectrum communicationssystem, as recited in claim 1, wherein said second pseudo noise sourceprovides a fixed length code sequence.
 4. A spread spectrumcommunications system, as recited in claim 1, wherein said firstmultiplier mixer spreads said data from said data source with a variablePN code.
 5. A spread spectrum communications system, as recited in claim1, wherein said second multiplier mixer spreads said data from saidfirst mixer with a fixed length PN code.
 6. A spread spectrumcommunications system, as recited in claim 1, wherein said first matchedfilter further comprises a set of coefficients correlated to said secondpseudo noise source.
 7. A spread spectrum communications system, asrecited in claim 1, wherein said plurality of phase/frequency shiftersare offset from each other by one or more degrees/Hz.
 8. A spreadspectrum communications system, as recited in claim 1, wherein saidsecond matched filters are correlated to said first pseudo noise source.9. A spread spectrum communications system, as recited in claim 1,wherein said first and said second pseudo noise sources may be scaled tolonger or shorter fixed or variable code lengths to optimize variablessuch as connection quality, system complexity, overall implementationcost and service “classes” of devices.
 10. A spread spectrumcommunications system, as recited in claim 1, wherein saidequalizer/decoder selects an advantageous set of signals from saidreceived signals from said plurality of phase/frequency shifters.